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Cotton: manage weeds, manage mites

Managing spider mites in cotton fields starts with early-season weed management of fields and field borders, according to John Smith, Extension associate in entomology at Mississippi State University.

Spider mites were the third most damaging pest in Mississippi cotton during 2007 and 2008 behind plant bugs and bollworms. Losses in 2009 likely would have been higher if near record rainfall had not occurred across the state. Nationally, spider mites were the sixth ranked pest in 2009, infesting 2 million acres and causing the loss of 22,000 bales.

Spider mites use a number of hosts around cotton fields and can remain active in the winter, said Smith at the Mississippi Crop College, at the Delta Research and Extension Center in Stoneville.

“They also have a short development time, in the summer as short as five to seven days. This allows them to build populations very quickly and produce multiple overlapping generations. Without mortality, in just over a month, the progeny from one spider mite can number in the millions. We typically don’t see that, due to environmental effects and predation. But it is possible for spider mites numbers to build very quickly if we kill beneficial and flare mites, or we get hot, dry conditions.”

To manage mites, Smith recommends that producers start with a clean field and field borders. “Usually, when we see mites on seedling cotton, there are weeds around. So managing the weeds should help manage the mites as well.”

Research on the overwintering habits of spider mites indicates that a large percentage of the population remains active during the winter.

“At any point in the winter we sampled, we could find eggs and immature mites. This means populations can build early in the spring. In February and March, populations are really increasing, and they can spread across our fields early on weeds that are not controlled.”

The most important host plant for spider mites is henbit, Smith says. “We think one reason is because it gives them protection from the rain. Other hosts include geraniums and cutleaf evening primrose. A lot of the broadleaf weeds are good hosts for mites. The grasses are not good hosts with the exception of johnsongrass.”

Studies on the timing of burndown and herbicide selection on spider mite populations found that neither aspect was critical “as long as all the weeds were killed well before planting cotton. Where we did not have an effective burndown, the spider mites jumped on at the cotyledon stage and killed a lot of cotton.”

The increase in spider mite pressure in Mississippi roughly coincides with the switch from in-furrow insecticides to seed treatments, Smith noted. He cited a recent study in North Carolina which showed that Temik applied in-furrow at planting reduced the odds of having to spray mites from 5 percent to 0.09 percent, a nine-fold decrease in the odds.

Trials were also conducted in Mississippi comparing the number of spider mites for neonicotinoid seed treatments, Temik applied in-furrow and check plots. All the plots were artificially infested with spider mites. “Mites were usually about two-fold higher, one, two and three weeks after infestation with seed treatments versus untreated plots and plots treated with Temik in-furrow. We don’t know what’s behind it, it could be a variety of factors.”

However, researchers do not recommend a switch to Temik because of the research, Smith says. “Mites are really just a problem on seeding cotton on a small amount of acres. And mites can be controlled by controlling hosts weeds. On the other hand, if the field has high mite risk, Temik may be an option.”

Smith noted that in the absence of insecticides, “beneficial insects will keep most spider mite infestations in check. When we spray insecticides, we also kill beneficial insects and release mites. It’s usually when we start spraying that we start to see problems.”

Smith added that acephate and pyrethroids are the most likely insecticides to flare spider mites.

The threshold for treatment of mites in Mississippi is 30 to 50 percent infested plants with populations increasing and weather conducive for populations to increase. Smith recommends managing for spider mites out to 600 to 650 heat units past cutout. “However, we get a lot of calls every year, especially late in the year, when budgets are getting tight and questions arise about whether to treat rising populations. We don’t always have an answer.”

At least some of this is due to researchers not feeling comfortable with knowing how much damage the pests can cause.

Smith pointed to a study conducted in 1968 “which found that squaring and blooming infestations of spider mites caused 30 percent to 35 percent yield loss. However, they did not see a yield loss for late infestations.

“Another study conducted in Australia reported early-season yield losses of up to 78 percent for spider mites, mid-season losses up to 29 percent and late-season losses of up to 16 percent. The study also indicated that mites no longer affected yield after 20 percent open bolls. It tells us that we need to manage mites later in the year.”

A study in Stoneville found that an infestation of spider mites at the third true leaf caused a 40 percent yield loss. In another location in Louisiana, third true leaf infestations also resulted in a 40 percent yield loss. “There were also significant yield losses out to 400 heat units past bloom, up around 10 percent o 15 percent. We feel like these losses could be greater during a dry year.”

Smith believes spider mites appearing late season “could be coming out of corn and other crops, especially corn when it starts to dry down late in the year.”

Spider mites can be knocked out by a naturally-occurring fungus similar to the fungus which knocks out aphids. This usually requires cool, rainy weather with high humidity for two to three days. “We had the fungus hit Starkville, Miss., last year and we went from really high mite numbers all over the field to zero three days later. The fungus will zero populations out. But it’s hard to tell whether we can depend on this year after year. It doesn’t always show up. When they get the fungus, the mites will take on a cloudy appearance and will die within a day or two.”

Most miticides perform similarly on spider mites, noted Smith. “They don’t usually zero the numbers out, but they knock them way back, especially at the higher rates.”

Research has also indicated that the herbicide Ignite at the 29-ounce rate “has some activity on spider mites. It looks like a mid-rate of our standard insecticides. We don’t know the exact mode of action.”